A Wavelength Selective Switch (WSS) is a device used in Reconfigurable Optical Add Drop Multiplexers (ROADMs) in fiber optic telecommunication networks to route optical wavelength channels. Other uses of WSS are contemplated. FIG. 1 illustrates an example 1×4 WSS consisting of a single input port receiving an optical signal comprising wavelength channels (A, B, C, D, E, F) and 4 output ports. The magnitude of each wavelength channel at input and output is represented by the height of its respective column. Through control signals to the WSS, each wavelength channel from the input signal can be dynamically switched or routed to any one of the output ports, independent of how all other wavelength channels are routed.
Within a WSS, an input port receives input light comprising multiplexed wavelength channels. Imaging optics such as diffraction gratings, cylindrical lenses, spherical lenses and other components collimate and spatially disperse different wavelength channels onto a switching engine. The switching engine comprises an array of switching elements, each element of which receives one of the spatially dispersed wavelength channels and imparts to it a programmable tilt. The switching engine may be, for example, an array of tilting microelectromechanical systems (MEMS) mirrors, or a phased array device such as a Liquid Crystal on Silicon (LCOS) pixel array. After each channel has been tilted by the switching engine, imaging optics re-multiplex the wavelength channels and direct them to one of several output ports according to the tilt imparted by the switching engine.
One goal of a WSS is to achieve high port isolation. In an ideal system, perfect port isolation prevents any signals from unselected channels being collected at an output port. Conversely, each output port only receives signals from its selected channels. Thus, to achieve high port isolation, a WSS attempts to direct wanted diffraction orders at selected output ports while preventing unwanted diffraction orders from being received at non-selected output ports. The output signals illustrated in FIG. 1 figuratively demonstrate high port isolation because, at each output port, the magnitude of the selected channel or channels (identified by letter) is much greater than the magnitude of the unselected channels.
High port isolation is not easily achieved in a WSS. When each wavelength channel is diffracted or reflected by a phased array switching engine, multiple diffraction orders are generated and disperse at different angles from the switching engine. The presence of multiple, potentially overlapping, diffraction orders from each wavelength channel within a WSS can significantly decrease port isolation if enough unwanted diffraction orders are received at an output port that is not selected to receive that particular wavelength channel.
Referring collectively to FIGS. 2A, 2B, 2C and 2D, the potential overlap of diffraction orders in a WSS is illustrated. FIGS. 2A and 2B illustrate a simplified prior art WSS where the switching engine 10 is normal to the incident light 12, the optical components 14 have been abstracted, and the incident light 12 comprises one wavelength channel and WSS attempts to steer and collect first order (+1) diffractions.
FIG. 2A illustrates that the switching engine 10 can be configured to steer a wanted diffraction order from incident light 12 to be collected at any of the output ports (O1 through O5). FIG. 2A illustrates potential angles of wanted diffraction orders but does not illustrate the unwanted diffraction orders. FIG. 2B illustrates the unwanted diffraction orders (hashed lines) when the switching engine is configured to steer incident light 12 for coupling at output O3. Accordingly, O3 is the selected output port for the diffracted light while O1, O2, O4 and O5 are unselected ports which should not receive any diffracted light to achieve high port isolation. Unfortunately, high port isolation is not achieved in this example because the unwanted diffraction orders are collected at non-selected output ports.
FIGS. 2C and 2D generalize the problem identified in FIG. 2B for all output angles of the switching engine 10. FIG. 2C illustrates the overlap between possible angular ranges of 1st and 2nd diffraction orders from the switching array 10. FIG. 2D illustrates the overlap of those angular ranges when collected at the output ports. Without other measures, the WSS of FIGS. 2A, 2B, 2C and 2D cannot achieve high port isolation because the desired (+1) diffraction order steering angle range overlaps with unwanted orders.
Previous attempts to achieve high port isolation in a phased array WSS by reducing coupling of unwanted diffraction orders to non-selected ports typically follow two general approaches, in combination or separately.
The first approach increases WSS design complexity to increase port isolation. Under this approach, the phase profile of the switching array is customized to maximize the efficiency of receiving wanted diffraction orders relative to receiving unwanted diffraction orders. This has been achieved, for example, by overdriving an LCOS at the edges of its phase resets in order to reduce the width of the phase reset regions. With complex calibration and control techniques, the switching engine can be configured so the intensity of unwanted orders is low compared to the intensity of wanted orders. FIG. 3A illustrates a graph of phase change versus array position used to drive a switching engine under this approach. The horizontal axis (array position) represents linear cell position on the switching array. The vertical axis (phase change) represents the phase tilt imparted to incident light at that cell position in the switching array. The hashed line illustrates a common modulo 2π phase profile. Conversely, the solid line illustrates an example optimized phase profile. Providing dynamic control of a customized phase profile of a phased array switching engine is difficult and requires complex calibration and control techniques to achieve port isolation that is greater than 40 dB when an unwanted diffraction order is directed to a non-selected port. These complex, dynamic calibration and control features make using this first approach undesirable.
The second approach increases port isolation by increasing the size of the output optical aperture of the WSS. The output optical aperture defines an angular region across which the output signals are collected. To maximize the use of the optical aperture of a WSS and minimize physical size, output ports would ideally be separated only by a minimum angular spacing θ, which approximates the angular width of a wavelength channel's beam. This minimum spacing is necessary to avoid a different problem: adjacent port crosstalk. To increase port isolation, this second WSS approach significantly increases the output optical aperture by adding dead zones or empty regions between successive WSS output ports where unwanted diffraction orders may be directed so they are not received by unselected output ports. This approach makes inefficient use of the output optical aperture and undesirably increases the physical size of the WSS.
FIG. 3B illustrates example output ports of the second approach. Two spatially dispersed wavelength channels (A, B) are incident to a phased switching array within a WSS. The two first order diffractions of both channels are illustrated. One of the first order diffractions is collected at an output port while the other is directed at a dead zone between the two output ports. A “dead zone” is a portion of the output optical aperture that does not collect output signals. In effect, interspacing output ports with dead zones decreases the density of output ports and increases the size of the output optical aperture of the WSS.
The configuration in FIG. 3B accounts only for the 1st order diffractions. As shown in FIG. 2B, other diffraction orders may also adversely affect port isolation. To avoid coupling of unwanted 2nd order diffractions into non-selected ports, the interleaved port arrangement in FIG. 3B could space output port angles at ±1θ, ±3θ, ±5θ, etc. and interleave dead zones at ±2θ, ±4 θ, ±6θ, etc. To also avoid 1st order coupling against the blaze direction, the output ports could be interleaved at +1θ, −2θ, +3θ, −4θ, etc. and interleave dead zones at −1θ, +2θ, −3θ, +4θ, etc. As more unwanted diffraction orders are considered, port isolation increases; however, the number of dead zones and the size of the output optical aperture increase as well. Accordingly, the second approach is undesirable because it requires increasingly inefficient use of the output optical aperture to improve port isolation.
These two approaches respectively result in complex switching array control and calibration to suppress unwanted orders, and inefficient use of optical aperture due to unused output angle ranges. Accordingly, there is an unmet need for a WSS that achieves high port isolation by efficiently using its optical aperture without complex calibration and control of the switching array.